Amount and Vertical Distribution of Soil Organic Carbon and Total Nitrogen in a Dry Tropical Forest Ecosystem, Tanzania

Abstract

There is a growing interest in understanding soil organic carbon (SOC) and total nitrogen (TN) of various ecosystems worldwide, because they are important indicators of soil quality and soil fertility, especially on the availability of essential nutrients for plant growth; and climate change mitigation. We tested the hypothesis that the amount and vertical distribution of SOC and TN in 0-30cm and 30-100cm depths differ significantly in miombo woodland ecosystems. Soil samples were collected from 15m-radius circular plots (n=33). SOC was determined by Mid-Infrared (MIR) spectroscopy (ICRAF approach) and the Walkley and Black method (NAFORMA approach). The mean amount of SOC and TN at 30-100cm depth were significantly higher (<i>p</i>=0.003 and <i>p</i>=0.0001, respectively) than that within the 0-30cm depth. The amount of SOC at 20-40cm (39tCha^-1) was found to be significantly (<i>p</i>=0.0007) higher than at 0-20cm (32tCha^-1) followed by decreasing pattern to 100cm. On the other hand, TN decreased substantially from 0-20cm to 100cm depth. SOC was significantly (<i>p</i><0.05) and positively correlated with TN. The NAFORMA approach estimated significantly (<I>P</I><<i>0.05</i>) higher SOC than ICRAF approach. Clearing of forests for sesame cultivation invariably resulted in increased nitrogen in the top soil due to addition of ammonium fertilizers, but loss of SOC is due to removal of biomass (including slash burning) and a reduction in the quantity and quality of organic inputs added to the soil. Accurate estimation of SOC at national and regional scales should use the modern methods complimented by the standard methods in different ecosystems.